The present invention relates to novel polymers and in particular to novel copolymers having a broad molecular weight distribution, toughness and improved processability.
In recent years there have been many advances in the production of polyolefin copolymers due to the introduction of metallocene catalysts. Metallocene catalysts offer the advantage of generally higher activity than traditional Ziegler catalysts and are usually described as catalysts which are single-site in nature. Because of their single-site nature the polyolefin copolymers produced by metallocene catalysts often are quite uniform in their molecular structure. For example, in comparison to traditional Ziegler produced materials, they have relatively narrow molecular weight distributions (MWD) and narrow Short Chain Branching Distribution (SCBD). Although certain properties of metallocene products are enhanced by narrow MWD, difficulties are often encountered in the processing of these materials into useful articles and films relative to Ziegler produced materials. In addition, the uniform nature of the SCBD of metallocene produced materials does not readily permit certain structures to be obtained.
An approach to improving processability has been the inclusion of long chain branching (LCB), which is particularly desirable from the viewpoint of improving processability without damaging advantageous properties. U.S. Pat. Nos. 5,272,236; 5,278,272; 5,380,810; and EP 659,773, EP 676,421, relate to the production of polyolefins with long chain branching.
Another approach is the addition of the polymer processing aids to the polymer prior to fabrication into films or articles. This requires extra processing and is expensive.
A different approach to the problem has been to make compositions which are blends or mixtures of individual polymeric materials with the aim being to maximize the beneficial properties of given component while minimising its processing problems. This also requires extra processing which increases the cost of materials produced. U.S. Pat. Nos. 4,598,128; 4,547,551; 5,408,004; 5,382,630; 5,383,631; and 5,326,602; and WO 94/22948 and WO 95/25141 relate to typical blends.
Another way to provide a solution for the processability problems and to vary SCBD has been the development of various cascade processes, where the material is produced by a series of polymerizations under different reactor conditions, such as in a series of reactors. Essentially, a material similar in some ways to a blend is produced, with a modality greater than one for various physical properties, such as the molecular weight distribution. While polyolefin compositions with superior processability characteristics can be produced this way, these methods are expensive and complicated relative to the use of a single reactor. Processes of interest are disclosed in U.S. Pat. No. 5,442,018, WO 95/26990, WO 95/07942 and WO 95/10548.
Another potentially feasible approach to improving processability and varying SCBD has been to use a multicomponent catalyst. In some cases, a catalyst which has a metallocene catalyst and a conventional Ziegler-Natta catalyst on the same support are used to produce a multimodal material. In other cases two metallocene catalysts have been used in polyolefin polymerizations. Components of different molecular weights and compositions are produced in a single reactor operating under a single set of polymerisation conditions. This approach is difficult from the point of view of process control and catalyst preparation. Catalyst systems of interest are disclosed in WO 95/11264 and EP 676,418.
WO96/04290 teaches the use of the preferred metallocene complexes of this invention to make ethylene copolymers. In particular, Examples 44 and 45 teach the preparation of polymer using gas-phase techniques. The examples teach only operation for a hour or less in batch mode and no details of the original polymer bed composition is given.
U.S. Pat. No. 5,462,999 and U.S. Pat. No. 5,405,922 teaches the preparation of ethylene copolymers in the gas-phase using a silica supported metallocene catalyst. It is believed, however, that the products produced by following the examples will not contain long chain branching and in particular will have lower values for the parameters xcex4(MS)/xcex4(P) and xcex4(MS)/xcex4(log {dot over (xcex3)}) than is claimed herein:
EP 676421 also teaches the preparation of copolymers in the gas phase using a supported metallocene catalyst. The products produced in the examples of this patent in general also have lower values for the parameters xcex4(MS)/xcex4(P) and xcex4(MS)/(log {dot over (xcex3)}) than is claimed herein.
EP 452920 and EP495099 teach the production of ethylene copolymers using metallocene catalysts. Once again it is believed that the examples contained therein will not produce products with some or all of the desirable characteristics mentioned below
It would be desirable to be able to produce a polyolefin copolymer composition which is very easy to process and which is produced using a single metallocene catalyst system preferably supported in a polymerisation process using a single reactor, preferably gas phase, operating semi-continuously or, preferably, continuously under a single set of reactor conditions.
It would also be desirable to produce polymers which have the processability and impact strength similar to highly branched low density polyethylene (LDPE).
It would also be highly desirable to produce polymers having the above properties which may be suitable for use in low density polyethylene film applications.
We have now found copolymers of ethylene and alpha olefins may be prepared which have improved processability and which exhibit specific melt strength characteristics. Such copolymers are advantageously prepared using a single metallocene catalyst system using a single gas-phase, fluidised bed reactor.
Thus according to a first aspect of the present invention there is provided a copolymer of ethylene and one or more alpha olefins containing from three to twenty carbon atoms said copolymer having:
a) a long chain branching gxe2x80x2 value of less than or equal to 0.9 and
b) a value of the derivative function xcex4(MS)/xcex4(P) of greater than 0.6
wherein MS is the melt strength of the copolymer in cN and P is the extrusion pressure of the copolymer in MPa.
In a second aspect of the invention there is provided a copolymer of ethylene and one or more alpha olefins containing from three to twenty carbon atoms said copolymer having:
a) a long chain branching gxe2x80x2 value of less than or equal to 0.9 and
b) a value of the derivative function xcex4(MS)/(log {dot over (xcex3)}) of greater than 7.5
wherein MS is the melt strength of the copolymer in cN and {dot over (xcex3)} is the shear rate of the copolymer in secsxe2x88x921..
Also provided by the present is a homopolymer of ethylene or a copolymer of ethylene and one or more alpha olefins containing from three to twenty carbon atoms said homopolymer or copolymer having:
a) a value of the derivative function xcex4(MS)/xcex4(P) of greater than 0.6 and
b) an Mw/Mn value of in the case of the copolymer less than 8 and in the case of the homopolymer less than 6
wherein MS is the melt strength of the copolymer or homopolymer in cN and P is the extrusion pressure of the copolymer or homopolymer in MPa and Mw/Mn is the ratio of weight average molecular weight to number average molecular weight of the copolymer or homopolymer as measured by gel permeation chromatography.
The derivative function value xcex4(MS)/xcex4(P) is preferably xe2x89xa70.75 and more preferably xe2x89xa70.8.
In a further aspect of the present invention as herein described there is provided a homopolymer of ethylene or a copolymer of ethylene and one or more alpha olefins containing from three to twenty carbon atoms said homopolymer or copolymer having:
a) a value of the derivative function xcex4(MS)/xcex4(log {dot over (xcex3)}) of greater than 7.5 and
b) an Mw/Mn value of less than 6.5
wherein MS is the melt strength of the copolymer in cN and {dot over (xcex3)} is the shear rate of the copolymer in secsxe2x88x921.and Mw/Mn is the ratio of weight average molecular weight to number average molecular weight as measured by gel permeation chromatography.
Another aspect of the present invention is provided by a homopolymer of ethylene or a copolymer of ethylene and one or more alpha olefins containing from three to twenty carbon atoms, said homopolymer or copolymer having a long chain branching gxe2x80x2 value of between about 0.6 and about 0.9. The homopolymers and copolymers of this aspect of the invention may also have either or both of (a) a value of the derivative function xcex4(MS)/xcex4(P) of greater than 0.6 or (b) a value of the derivative function xcex4(MS)/xcex4(log {dot over (xcex3)}) of greater than 7.5, wherein MS is the melt strength of the copolymer in cN and P is the extrusion pressure of the copolymer in MPa and xcex3 is the shear rate of the copolymer in secsxe2x88x921.